This invention relates to power amplifiers, for example, the power amplifiers that include at least a two-dimensional matrix of transistors and can be tuned to match the impedance of an antenna or other load.
Transistors are devices that can amplify a control signal that is input into a control terminal. Transistors can be made from a variety of different materials, can have a variety of different geometries, and can operate according to a variety of different physical mechanisms. Example materials include silicon, gallium arsenide, gallium nitride, and silicon carbide. These and other materials can be used to form devices such as bipolar transistors and field effect transistors that include either insulated control terminals (e.g., IGBTs, MOSFETs, HEMT's or HFETs) or that include control terminals made from PN junctions (e.g., BJTs or JFETs).
Regardless of the materials and device structures, individual transistors all have fundamental limits on their safe operational ranges. For example, if an excessively large voltage is applied across the main terminals, then dielectric breakdown may occur and the transistor may be damaged or destroyed. As another example, if an excessively large current flows between the main terminals, then the transistor may also be damaged or destroyed.
Although the operational ranges of individual transistors may be suitable for some applications, they may be insufficient to meet the requirements of others. For example, some applications may require voltages in excess of the breakdown voltage or currents in excess of the peak currents of even well-designed transistors. Examples of such applications include driving antennas for the transmission of, e.g., radar signals and communication signals (e.g., for satellite communication and terrestrial broadcasts in both military and civilian contexts).
In such applications, individual transistors can be grouped in order to handle large voltages and/or currents as a group. For example, individual transistors can be stacked (or “series-stacked”) so that almost all of the current that flows through the main terminals of the first transistor in the stack also flows through the main terminals of subsequent transistor(s) in the stack. Each of the transistors in the stack supports some portion of the voltage that drives this current. The total voltage supported across the stack of transistors can be in excess of the breakdown voltage of the individual transistors.
As another example, individual transistors can be paralleled so that essentially the same voltage is coupled across the main terminals of multiple transistors. When multiple transistors are conductive, the net current flow through the group can exceed the peak current of the individual constituent transistors.
In idealized small-signal models, power transfer from a signal source to a load can be improved by matching the output impedance of the source (also known as “the source impedance”) to the impedance of the load. In such models, the maximum possible power is transferred when the impedance of the load is exactly equal to the complex conjugate of the source impedance over an infinite range of frequencies.
However, in real-world, large-signal applications, the “source impedance” is not properly defined or, strictly speaking, does not exist due to lack of linearity or the missing superposition law. Nevertheless, theory and practice show that a sufficiently large portion of the power is transferred (i.e., the power-added efficiency (PAE) is sufficiently high) for certain values of the load impedance. The term “optimum load impedance” is used herein to characterize the circumstances where a sufficiently large portion of the power is transferred over a range of operational frequencies of interest.